Notes Receivable Backed by Real Estate - Summary of Notes Receivable Backed by Real Estate (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 $188 million Preferred Equity Interest [Member]	Dec. 31, 2011 $188 million Preferred Equity Interest [Member]	Dec. 31, 2012 $55 million Preferred Equity Interest [Member]	Dec. 31, 2012 NAIF II Secured Mortgage Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 322,834	$ 188,000	$ 188,000	$ 55,970	$ 78,864
Elimination upon acquisition of NAIF II	(78,864)				(78,864)
Principal payment received	(55,000)			(55,000)
Accrued interest, net of interest payments received	(970)			(970)
Ending balance	$ 188,000	$ 188,000	$ 188,000